Conventional and emerging technologies have needs for parts made of thin foil materials, particularly thin foil metal materials. It has been found that existing forming operations are unable to cost-effectively form thin foil materials into parts having desired shapes with simple and compound surfaces, and features such as wrinkle-free flanges.
For example, in the manufacture of thin foil trays suitable for use in vacuum insulation panels, such as shown in U.S. Pat. No. 2,745,173, issued May 15, 1956 to Janos, metal materials are desirable for use because of their strength and ability to seal for vacuum retention. However, this particular application requires substantially wrinkle-free flanges for vacuum tight sealing of the formed part to other parts. While thin foil materials would be desirable for reduced conductive heat leak across such insulation panels, it has been necessary to stamp thicker cold-rolled carbon steel sheet material to practice the invention of the '173 patent.
Conventional processes applied to produce thin foil metal material parts, such as trays, have limitations and drawbacks which make their use in commercial production problematic. Conventional processes include matched metal die stamping, thermoforming, hydroforming, and rubber pad forming.
For example, matched metal dies are expensive to machine, expensive to align for use, and require high clamping pressures. Insufficient clamp pressure or imperfect flatness between the two mating halves of the tool permits excessive motion of a thin foil workpiece into the forming tool, and results in a buckling mode type of failure of the foil which produces wrinkles. However, as some material draw is desirable, excessive clamping force does not solve the problem of wrinkling and further promotes tearing of thin foils during forming. In addition, matched metal dies produce shapes with non-uniform stress distribution which causes tearing in thin foils, particularly in corners. Some desirable results without wrinkling or tearing have been obtained with matched metal dies, but due to failure rates for foil materials, matched metal die processes are limited to thicker workpiece materials for economical production levels. Lubricants may be applied to enhance forming and reduce tearing of thin foil workpieces, but introduce contaminants and necessitate a post-application cleaning step, increasing production costs. However, wrinkling remains a problem even where lubricants are used.
Thermoforming of superplastic metal materials is a low pressure, high temperature process. However, foil materials are limited to conventional thermoplastic metal materials, such as certain alloys of magnesium, zinc and aluminum capable of elongation of approximately 500% or more. While lower forming pressures are enjoyed, in addition to limited material choices, higher temperatures and related die warping and energy costs, as well as increased cycle times due to heating, are additional significant drawbacks of thermoforming.
Hydroforming, by contrast, is a high pressure, standard or ambient room temperature process. However, practical considerations make difficult the hydroforming of parts having a surface area greater than about 18 inches by 18 inches. Moreover, higher failure rates, i.e. incidence of tearing and wrinkling, occur in hydroforming thin foil materials, even where the foil is sandwiched between cull plates. Cull plates are thicker pieces of steel formed along with the foil workpiece to protect it. However, the use of cull plates increases cycle time and forming pressures. As well, since the cull plates are formed along with the thin foil, they are not reusable and exact a cost penalty in production. Rubber pad forming has similar drawbacks to hydroforming, such as the need for cull plates, and higher failure rates.
Finally, because moderate to high forming pressures and clamping forces are required to form foil materials, some of these above-mentioned forming operations use elastomeric or resilient surfaces in compression with the foil workpiece. Hereafter, elastomeric or resilient surfaces will be referred to as resilient surfaces. Wherever clamping forces and forming pressures bring a foil workpiece and resilient surfaces together, air is expelled from between the two, much like during compression of a suction cup. Because thin foils are compliant, air cannot easily re-enter the tight space between the foil and the resilient surface. After the forming operation is complete, the foil is left firmly adhered to the resilient surface. The foil is often damaged during the process of its removal, and may require manual removal. This occurs whether large surface areas or annular or peripheral areas of the foil materials are compressed against the resilient surface.
Again, conventional forming operations such as hydroforming and rubber pad forming overcome these further difficulties by sandwiching the thin foil between cull plates which can withstand the peel back force typically encountered with rubber diaphragms and pads. However, as noted, these activities increase cycle time and production costs.
Accordingly, improvements in forming thin foil sheet materials are needed to produce more cost effective shapes and products using thin foil materials.